The present invention relates to an electric lamp which includes an end-of-life device that will safely extinguish the source of light when an outer lamp envelope is broken, without adversely substantially affecting lamp efficacy or providing an undesirable second light source during normal lamp operation. The present invention is particularly of interest regarding a tungsten halogen lamp.
A problem with some electrical lamps is that they present a fire hazard and may cause burns should the outer lamp envelope break. Although there is more than one type of lamp with respect to which this problem applies, the problem is particularly applicable to tungsten halogen lamps. Due to higher efficacy and coil temperatures, tungsten halogen lamps are attractive as replacements for ordinary incandescent filament lamps for general service lighting. However, tungsten halogen lamps typically operate at high bulb wall temperature and high internal pressure. For these reasons, tungsten halogen lamps are usually operated in fixtures with shielding to protect against accidental contact. The use of special fixtures increases the cost of using a tungsten halogen lamp and limits its usefulness to those applications where use of such a fixture is practical. In an alternative embodiment, it is known to seal the typical halogen capsule in an outer envelope having a thick wall that performs the shielding function. However, the use of thick walls increases the cost of the lamp and reduces the transmission of light. In addition, the thick walls of the lamp adds to its weight which can be a problem with some fixtures.
The use of a tungsten halogen lamp having a thin outer envelope is well know. However, such lamps present a potential safety hazard should the outer envelope break and the inner halogen capsule continue to operate. In particular, the wall temperature of the tungsten halogen inner capsule is high enough to cause burns and to ignite various materials such as paper and fabric. The safety problem is compounded due to the fact that such lamps may be intended as replacement lamps for general service incandescent lamps and therefore are likely to be used in a wide variety of lighting fixtures, some of which could easily be tipped over or damaged in such a way as to break the outer glass envelope exposing the still burning inner halogen capsule.
The use of other types of lamps involve problems similar to those of tungsten halogen lamps. For example, high pressure discharge lamps include an inner arc tube contained within an outer envelope. One of the functions of the outer envelope is to permit passage of visible light, and block ultraviolet radiation, emitted by the inner arc tube during use of the lamp. Should the outer envelope be broken, such ultraviolet radiation will be emitted out of the lamp into the environment proximate the lamp.
A number of ways have been disclosed to interrupt electrical current to an inner lamp in the event of damage to an outer lamp envelope. For example, it is known to provide a high intensity discharge (HID) lamp having a mechanical switch, the circuit provided thereby being opened when the outer lamp envelope is broken. An example of such a lamp is described in U.S. Pat. No. 4,229,678. One problem with a mechanical switch is that opening of the switch requires sufficient outer envelope breakage adjacent the switch. Pin point breakage or breakage at a spot remote from the switch will not cause the switch to open the circuit.
It is also known to provide an HID lamp having an oxidizable fuse in the outer envelope which oxidizes and interrupts the lamp circuit in the event the outer envelope breaks and admits air. One preferred fuse material is a self-heated metal wire or coil that is reactive in the presence of air. An example of a lamp which includes an oxidizable fuse within the outer envelope is described in U.S. Pat. No. 4,361,782. One problem with the use of an oxidizable fuse is that substantial power is consumed in the fuse resulting in reduced lamp efficacy. Another problem is that most suitable fuse metals require incandescent temperatures in order to burn in air and therefore present a secondary and non-aesthetic light source within the outer envelope during normal lamp use.
It is also known to provide a pressure switch within the outer lamp envelope, such pressure switch being designed to open the lamp circuit when exposed to normal atmospheric pressure after the outer lamp envelope breaks. An example of such a lamp is described in U.S. Pat. No. 4,143,301. One problem with the use of a pressure switch is that it is bulky and expensive.
It is an object of the present invention to provide an improved lamp.
It is another object of the present invention to obviate the disadvantages of the prior art by providing an improved lamp.
A further object of the present invention is to provide an economical, efficient and high quality lamp.
Another object of the present invention is to provide a lamp having an oxygen sensitive end-of-life device operable at low wattage.
Yet another object of the present invention is to provide a lamp having an end-of-life device that consumes a reduced amount of power, and therefor does not reduce lamp efficacy, during normal operation.
A further object of the present invention is to provide for extinguishing an inner lamp upon breakage of an outer envelope using an end-of-life device, yet reduce the power consumed by the end-of-life device during normal lamp operation to less than about 0.5% of nominal lamp power.
Yet a further object of the present invention is to provide a safe tungsten halogen lamp having a thin walled outer envelope.
Another object of the present invention is to provide for automatically extinguishing an inner lamp in less than 60 seconds, and preferably in less than 5 seconds, measured from breakage of an outer lamp envelope.
A further object of the present invention is to provide a lamp having a end-of-life device which does not present a secondary and non-aesthetic light source during normal lamp operation.
Yet a further object of the present invention is to provide an improved method of operating an electric lamp.
This invention achieves these and other objects by providing an electric lamp, which includes a sealed first envelope enclosing an environment and being transparent to light. A first electrical conductor and a second electrical conductor are sealed into and pass through the first envelope and are electrically connectable externally of the first envelope to a source of electrical power. An electric lamp capsule, contained within the environment, comprises a sealed second envelope, a first lead wire and a second lead wire sealed into and passing through the second envelope, the first lead wire being electrically connected to the first electrical conductor and the second lead wire being electrically connected to the second electrical conductor, to provide a lamp circuit. An end-of-life device is contained within the environment and is electrically connected in series with the lamp circuit. The end-of-life device is of the type that will rapidly ignite during lamp operation in the presence of air to open the lamp circuit. A filling is contained within the environment, the filling being inert at least relative to the end-of-life device. The filling has a thermal conductivity high enough to cool the end-of-life device during energization of the electric lamp capsule within the sealed first envelope sufficiently to lower resistance and power loss in the end-of-life device, thereby increasing lamp efficacy and eliminating visual radiation of the end-of-life device. A method of operating the electric lamp of the present invention is also described.